The invention relates to a spectrophotometer, more particularly, a spectroscope for taking out a monochromatic light having a specific wavelength in a spectrophotometer.
In a spectrophotometer, such as an ultraviolet visible light spectrophotometer and an atomic absorption spectrophotometer, a spectroscope (monochromator) has been used to obtain monochromatic light having a predetermined wavelength. The spectroscope has a structure generally including a diffraction grid (wavelength dispersion element) and a driving mechanism for changing an angle of the wavelength dispersion element with respect to an incident ray. The diffraction grid is suitably rotated by the driving mechanism to thereby take out monochromatic light of the desired wavelength through an exit slit, the position of which is fixed. Therefore, the precise wavelength of the monochromatic light is greatly dependent on the precise angle of the diffraction grid. Therefore, the driving mechanism for precisely rotating the diffraction grid with a micro-pitch is required in order to increase the wavelength precision.
Heretofore, generally, there has been known a driving mechanism using a sine bar mechanism. Also, there has been proposed a driving mechanism using a Harmonic Drive (registered trade mark) mechanism, which is a kind of reduction gear. While the Harmonic Drive mechanism is a good reduction device for obtaining a large reduction ratio, it is inevitable to generate a cyclic error from the characteristics of the structure. The cyclic error directly lowers the precision of the wavelength of the spectrophotometer.
Therefore, the applicant of the invention has proposed a method for correcting the error as disclosed in Japanese patent publication (KOKAI) No. 1-221627 (page 3, column on upper right, line 10—the same page, column on lower right, line 2). In the correction method proposed by the publication, the error shape due to the Harmonic Drive mechanism is measured in advance, and a formula approximate to the error shape is formed in advance. Then, the wavelengths of a plurality of the bright line spectrums are actually measured, and a parameter, the wavelength deviation of which becomes smallest, is determined by changing combination of the parameters contained in the approximate formula. With this correction, the cyclic error of the Harmonic Drive mechanism is considerably solved.
In case such a correction is carried out, as the number of the bright line spectrums, i.e. the number of the referring wavelengths, is increased, the precision of the approximate formula becomes higher. However, actually, since the number of the bright line spectrums is considerably limited, it is difficult to increase the precision of the approximate formula. Also, as disclosed in the publication, in case the cyclic error of the Harmonic Drive mechanism is measured in detail by using an accurate encoder or the like at the beginning and the approximate formula is formed based on the measured results, the adjusting work or the like on the manufacturing process becomes very complicated and takes large troubles, which may result in an increased cost.
In view of the above problems, the present invention has been made and an object of the invention is to provide a spectrophotometer employing the Harmonic Drive mechanism as a reduction mechanism for a spectroscope, wherein while suppressing increase in costs for manufacturing, assembling and adjusting works, the cyclic error of the Harmonic Drive mechanism is sufficiently corrected to thereby carry out an analysis at a high wavelength precision.
Further objects and advantages of the invention will be apparent from the following description of the invention.